A towering problem

Imagine it's a cool autumn evening and you are a small songbird winging southward after an exhausting breeding season in Canada. The hazards of the terrestrial world — hungry cats, window-skinned skyscrapers, careless drivers — have melted away. Up here, one thousand feet above the Earth, it's smooth sailing. South America — and its feast of insects — awaits.

Suddenly, a strange red light materializes in the distance. It's mesmerizing. You can't look away. Powerless against its luminous charm, you circle the beacon, again and again and again, until, with a jolt, you smack headfirst into a thick metal guy wire, and the world goes black.

In the 1950s, reports of birds killed at communication towers in North America — although common — were dismissed as a minor conservation matter. However, a new study published this week in the open access journal PLoS ONE, suggests that communication towers represent a much larger source of avian mortality than previously thought.

According the study, North America's 84,000 communication towers — structures used for radio and television broadcast — kill nearly 7 million birds every year. The birds, most of which are neotropical species traveling to or from their breeding grounds in Canada and the United States, are drawn to the towers' signal lights," said the study's lead author, Travis Longcore of the University of Southern California, in a EurekaAlert press release from the American Academy for the Advancement of Science. "In the presence of the solid red lights, the birds are unable to get out of their spell. They circle the tower and run into the big cables holding it up," Longcore says.

For birds, the lighted towers are especially alluring in bad weather, when clouds force the birds to fly low and obscure the stars and moon, birds' natural navigation aids.

The study's authors found that the most troublesome towers are the tallest; they require a mess of support cables and project higher into the birds' flight path than shorter, free-standing towers. "The tallest 1.9 percent of towers account for 71 percent of the mortalities," claimed the press release accompanying the study. What's more, the study's authors determined that towers equipped with blinking red lights rather than steady-burning lights cause fewer bird deaths.

In the U.S., the areas most associated with tower-related avian mortality are the Gulf and Southeastern coastal regions. Meanwhile, the Rocky Mountain, Pacific Northwest, and Desert Southwest regions, had the lowest incidence of tower-related bird deaths — a possible function of the limited number of communication towers and nocturnal bird migrants in those areas.

And while 6.8 million dead birds a year may seem like a lot, communication towers aren't nearly as dangerous for avifauna as domestic and free-roaming cats — which kill more than 100 million birds every year — or bird/window collisions, which account for a billion bird deaths annually (for more, see HCN's infographic on avian hazards).

Still, bird death at communication towers is "a tragedy that does not have to be," says Longcore in the release. "[O]ne of the things this country has been great about is saying we care about not losing species on our watch. With these towers, we are killing birds in an unnatural way. This is senseless."

The study's authors estimate that replacing steady-burning signal lights with blinking lights on towers over 450 feet tall — there are more than 4,000 in North America — would save the lives of 2.5 million birds every year. The authors also suggest that businesses reduce the number of guyed towers and communication towers in general by building free-standing towers that don't need as many support cables and sharing towers.